1. Field of the Invention
This invention relates generally to probes and more specifically to probes adapted for use in testing microcircuits commonly associated with miniaturized electronic components sometimes referred to as chips.
2. Description of the Prior Art
Apparatus for probing and testing electrical circuits has typically included a base member and a table movable in two horizontal directions relative to the base member. A chuck supporting the electrical circuit or chip being tested has been carried by this table and has been actuatable to move the chip relative to the table in a generally vertical direction. A plurality of probes have each had a support member attached to the base member. Each of the probes has further included a probe arm having a fixed end attached to the associated support member and a free end extending into proximity to the chip being tested. A probe tip has been disposed at the free end of each of the probes. With the raising of the chip by the chuck, a predetermined point on the chip has been brought into contiguous relationship with an associated one of the probe tips to test the associated circuit.
As noted, the movement of the chip relative to the tips of the probes has typically been accomplished by providing the table with a chuck for raising the chip into contact with the probe tips. The movement of the chip to contact the probes has been relatively slow due mainly to the combined mass of the chuck and the chip which is relatively great as compared to the mass of the probes. Further, the use of sliding members which undergo wear with continued usage has been required in providing vertical movement of the chuck and chip relative to the table.
It is generally desirable that all of the probe tips contact the associated predetermined points on the chip with approximately the same probing force. Then by carefully adjusting the probing force, one can ensure that all of the probes make contact with the chip and further ensure that none of the probes damage the circuit being tested. In the past, the probe arms have been provided with spring characteristics so that the probing force has been dependent upon the deflection of the associated probe arm. In order to provide a substantially uniform probing force, it has been desirable to provide the probe arms with the same spring constant and then to ensure that the probes deflect the same distance. To provide a constant deflection distance, it has been desirable to planarize the probe tips; in other words, adjust the probes so that the probe tips lie in a single plane. This planarization requires very fine adjustments which have been difficult to make. Furthermore, the probe tips must not only be planarized with respect to each other but also with respect to the plane of the chip.
The deflection of the spring arms has also been dependent upon the particular point at which the probe tip contacts the chip. If the chip surface is irregular, the deflection will vary depending upon whether the probe tip contacts a peak or a valley on the chip surface.
One means of providing a substantially constant probing force has been to preload the spring probing arms. This has been accomplished by biasing the probing arms in the direction of the chip. The probing force is then equal to the preload force, plus any additional force produced by deflection of the probe arms. Since the preload force is a large percentage of the total probing force, it is necessary that the probes be planarized so that all of the probes will receive the preloading force.
With preloading of the probe arms, the spring constant and preloading characteristics of the probe arms have heretofore been set by the manufacturer. This has not left the user of the testing apparatus with sufficient control over the magnitude of the probing force since the user has not been able to independently vary the preloading of the probe arms.
Most of the probes of the prior art have either used a single probe arm or a pair of probe arms of equal length. In both types of probes, the raising of the chip in contact with the probe tip has caused the probe tip to deflect along an arcuate path. This result, which is commonly referred to as physical arcing, has tended to scratch and therefore physically damage the electrical circuit being tested.
In some prior art testing apparatus, the chip has been moved in a plane substantially parallel to the probe tips with the probes being moved into contact with the chip. In one apparatus, piezoelectric crystals have been provided in the probe arms and have been energizable by electrical signals to deflect the probe arms into contact with the chips. A problem in this type of probe is that the piezoelectric crystals may warp during deflection so that the accuracy of contact between the probe tip and the chip has been difficult to maintain.
Also, piezoelectric crystals are energized by electrical signals which create electrostatic and magnetic fields. Such fields are undesirable since they may tend to damage metal-oxide-silicon semiconductors which may form the chips being tested. Furthermore, the piezoelectric crystals used in the probe arms of the prior art have been fragile with the result that the probe arms may be easily damaged.